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Asset Retirement Obligations (ARO) (Tables) | 6 Months Ended
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Jun. 30, 2016
Asset Retirement Obligation Disclosure [Abstract] |
Reconciliation of Beginning and Ending Aggregate Carrying Amount of Asset Retirement Obligation | A reconciliation of the beginning and ending aggregate carrying amount of the ARO is shown in the following table.
(Thousands of dollars) |  | June 30, 2016 |  | December 31, 2015
Balance at beginning of period |  | $ | 24,345 |  |  | $ | 22,245 |
Accretion expense |  | 825 |  |  | 1,521 |
Liabilities incurred |  | 6 |  |  | 579 |
Settlement of liabilities |  | $ | (164 | ) |  | $ | — |
Balance at end of period |  | $ | 25,012 |  |  | $ | 24,345 |

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- Definition Tabular disclosure of the changes in carrying amount of a liability for asset retirement obligations, for changes such as new obligations, changes in estimates of existing obligations, spending on existing obligations, property dispositions, and foreign currency translation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13201-110859 + Details
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